Phytoestrogens, present in soy-based products, are being consumed by humans in increasing quantities; particularly by women tasking soy isoflavones concentrates for relief of menopausal symptoms. The long-term health effects of this exposure are not known, but because phytoestrogens likely share many of the biological effects of endogenous and synthetic estrogens, they might share their detrimental as well as beneficial effects. The pharmacology of estrogens is complex, however, and not all estrogens have the same profile of beneficial vs. detrimental activities as does estradiol, the endogenous estrogen. Estrogen action is also mediated by two estrogen receptors, ERalpha and ERbeta, proteins that have different tissue distributions and regulated somewhat different sets of genes. ERalpha and ERbeta also respond differently to subtype-selective ligands, an issue of importance because phytoestrogens have a more distinct preference for ERbeta than estradiol. This Project is part of a Research Program Project whose overall goal is to evaluate the long-term health effects that phytoestrogen exposure might have in aging women, using appropriate animal models for breast cancer, lipid metabolism, and cognition. The aims of this Project are: (1) to characterize on a genome-wide basis the pattern of genes that are regulated by the various phytoestrogens through ERalpha and ERbeta in target cells and tissues, and how this pattern compares with regulation by estradiol and other estrogens, (2) to define how the phytoestrogens (as opposed to estradiol and other estrogens) bind to and regulate the conformation and coregulator interaction of the two estrogen receptors, ERalpha and ERbeta, including identifying the specific co-regulator proteins through which these phytoestrogen-receptor complexes than act at specific promoter sites, using novel fluorescence methods developed in our laboratories, and (3) to use novel pharmacological tools, namely specific agonists and antagonists of ERalpha and ERbeta developed in our laboratories, as defined hormonal probes to assess the balance of phytoestrogen action through the two ER subtypes. The results of this project should provide important information on the manner in which phytoestrogens regulate gene expression through ERalpha and ERbeta, in comparison with that of other natural and synthetic estrogens. This information will be useful in evaluating the long-term health effects of phytoestrogen exposure in aging women.